1. Technical Field
The present invention relates to the field of fiber optics and, more particularly, to an optical fiber detection system which detects the occurrence of disturbances and breaks in an optical fiber loop.
2. Description of the Related Art
Distributed optical fiber sensing or detection systems have been developed which include low-loss optical fibers and solid state laser light sources. Such systems may be comprised of a single fiber sensing loop which extends kilometers in length.
Optical fiber sensing loops have been used in indoors and outdoors security applications. For example, sensing loops have been installed in floors and walls, and under carpets. Regarding outdoors applications, optical fiber sensing loops have been installed under surfaces traversed by people and vehicles, about the perimeter of secured areas to detect unauthorized entries, and in telecommunication and power cables to detect tampering with the cables.
The known optical fiber sensing systems have been mostly based on the Sagnac and Mach-Zehnder interferometers. Interferometers monitor phase differences between two optical signals, whose phases have been shifted by changes in the optical properties of their respective light paths. Optical fibers are sensitive to physical disturbances caused by pressure and temperature changes. Such disturbances affect the optical properties of the fiber light path. The changes in the optical properties of the fibers may be in the form of an elongation, a change in the index of refraction, birefringence, or a combination of these and other related optical effects.
U.S. Pat. No. 4,885,462 to Dakin discloses an optical fiber sensing system comprised of a Sagnac interferometer and a Mach-Zehnder interferometer. The primary objective of the system is to detect the occurrence and location of disturbances to the system.
Another optical fiber sensing system is disclosed in U.S. Pat. No. 5,355,208 to Crawford et al. The system comprises a Sagnac interferometer sensing loop and associated electronic components. The system detects the occurrence and location of remote disturbances.
A significant disadvantage of the known phase sensitive detection systems is their inability to reliably and accurately detect the occurrence of breaks in optical fiber loops. An interferometer may be used, for example, to activate an alarm if a zero or near zero DC level is measured in a fiber loop to indicate the occurrence of a break. The DC level of an interferometer may go to near zero due to natural fluctuations in the light signal, however, without the fiber loop actually being cut or broken. Thus, although interferometers are able to detect the occurrence of a break, they are also prone to give false readings of breaks when none have actually occurred.
A further disadvantage of the known phase sensitive detection systems is that an ON/OFF-type detection system cannot be implemented in the systems, to detect the occurrence of breaks in the sensing loop, without introducing operational problems into the modified system. More particularly, ON/OFF type detection systems detect changes in the intensity of the light signal, from an ON level to an OFF level, due to a break occurring in the sensing loop. The ON/OFF-type detection systems do not detect phase differences between light signals.
An attempt to implement an ON/OFF detection system in an interferometer may cause the interferometer to trigger an alarm even without the occurrence of a break in the loop, due to the phase-bias effect in the system.
Therefore, there has been a need for an optical fiber detection system which is capable of positively detecting the occurrence of disturbances and breaks in a sensing loop, and is not adversely affected by optical phenomena within the sensing loop.